1. Technical Field
The inventive arrangements relate generally to video recording systems and more particularly to video recording systems that record digitally encoded video sequences onto disc media such as recordable digital video discs, hard drives and magneto optical discs.
2. Description of Related Art
MPEG video generally uses three types of picture coding methods: Intra (I) pictures, predictive (P) pictures and bidirectional predictive (B) pictures. I pictures are encoded or decoded independently of any other picture. This creates a reference picture from which P and B pictures, or non-I pictures, can be constructed.
A number of MPEG video signals, however, are encoded without I pictures. In particular, many U.S. cable systems broadcast MPEG signals that do not contain I pictures. At first glance, such a video signal appears impossible to decode since there are no I pictures from which to construct the P and B pictures.
Nevertheless, a video signal without any I pictures can be decoded by most MPEG decoders because a separate portion of each P picture in the signal is typically composed of I macroblocks. That is, successive P pictures containing I macroblocks can be used to eventually properly decode a P picture, which can then be used to decode the remaining pictures in the video signal. As an example, in a block of five P pictures, twenty percent of each P picture can contain I macroblocks. For example, the top twenty percent of the first P picture can be composed of I macroblocks and the lower eighty percent can be composed of non-I macroblocks. Referring to the second P picture in the video signal, the portion representing the twenty percent of the picture immediately below the top twenty percent can comprise I macroblocks while the lower sixty percent and the top twenty percent can be composed of non-I macroblocks. Thus, a different portion of each successive P picture contains I macroblocks. Hence, the bottom twenty percent of the last P picture can contain I macroblocks.
These I macroblocks, along with the non-I macroblocks contained in the P pictures, can be used to assemble each successive P picture. Specifically, as each P picture is decoded, the I and non-I macroblocks can be stored in memory. As such, the decoder can generally properly decode the fifth P picture, from which the remaining P and B pictures can be decoded. During normal playback of a video signal having no I pictures, there is a brief period in which the picture quality suffers at the initiation of the playback. This is because the pictures at the beginning of the playback must be constructed from the P pictures that are not yet properly decoded. As an example, the first P picture in the playback signal normally contains the first portion of I macroblocks. Thus, the P and B pictures that are constructed from the first P picture cannot be properly decoded, as the first P picture contains only roughly twenty percent of the information needed to produce these pictures. As the playback continues, however, the picture quality improves since more of the P pictures are decoded thereby providing a greater number of correctly decoded I and non-I macroblocks until a properly decoded P picture is acquired. This initial reduction in picture quality is acceptable since it is brief, as a properly decoded picture is usually constructed within the first one-half to one second of normal playback of the video.
Significantly, however, the delay in building a properly decoded P picture in a signal lacking I pictures may be prolonged during a trick mode. As an example, during a fast-forward trick mode, a plurality of pictures are skipped to speed up the playback. If P pictures containing the I macroblocks are skipped, then it will take longer to properly decode a P picture for decoding the remaining P and B pictures. As a result, a greater number of pictures constructed during the trick mode will suffer from a degradation in picture quality. Additionally, the delay in producing an acceptable playback increases for a faster playback speed, as a greater number of P pictures may be skipped to accommodate the faster playback. Slow motion playback can contribute to a longer delay in producing the properly decoded P picture as well. This delay is because pictures are typically repeated during slow motion playback. Since pictures are being repeated, the properly decoded P picture will take longer to create because there will be a delay in decoding the macroblocks from the five separate P pictures. As a result, the acceptable reduction in picture quality during normal playback may become unacceptable during trick mode operation. Thus, a need exists for a method and system of modifying video signals by converting non-I pictures into P pictures to overcome the various detriments described above.